src/resolve/resolved-resolv-conf.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <resolv.h>
   4 #include <stdio_ext.h>
   5
   6 #include "alloc-util.h"
   7 #include "dns-domain.h"
   8 #include "fd-util.h"
   9 #include "fileio-label.h"
  10 #include "fileio.h"
  11 #include "ordered-set.h"
  12 #include "resolved-conf.h"
  13 #include "resolved-dns-server.h"
  14 #include "resolved-resolv-conf.h"
  15 #include "string-util.h"
  16 #include "strv.h"
  17
  18 /* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
  19 #define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"
  20
  21 /* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
  22 #define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"
  23
  24 /* A static resolv.conf file containing no domains, but only our own DNS server address */
  25 #define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"
  26
  27 int manager_check_resolv_conf(const Manager *m) {
  28         const char *path;
  29         struct stat st;
  30         int r;
  31
  32         assert(m);
  33
  34         /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
  35          * PRIVATE_STATIC_RESOLV_CONF or PRIVATE_STUB_RESOLV_CONF. */
  36
  37         if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
  38                 return 0;
  39
  40         r = stat("/etc/resolv.conf", &st);
  41         if (r < 0) {
  42                 if (errno == ENOENT)
  43                         return 0;
  44
  45                 return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
  46         }
  47
  48         FOREACH_STRING(path,
  49                        PRIVATE_STUB_RESOLV_CONF,
  50                        PRIVATE_STATIC_RESOLV_CONF) {
  51
  52                 struct stat own;
  53
  54                 /* Is it symlinked to our own uplink file? */
  55                 if (stat(path, &own) >= 0 &&
  56                     st.st_dev == own.st_dev &&
  57                     st.st_ino == own.st_ino) {
  58                         log_warning("DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to %s "
  59                                     "which expects DNSStubListener= to be enabled.", path);
  60                         return -EOPNOTSUPP;
  61                 }
  62         }
  63
  64         return 0;
  65 }
  66
  67 static bool file_is_our_own(const struct stat *st) {
  68         const char *path;
  69
  70         assert(st);
  71
  72         FOREACH_STRING(path,
  73                        PRIVATE_UPLINK_RESOLV_CONF,
  74                        PRIVATE_STUB_RESOLV_CONF,
  75                        PRIVATE_STATIC_RESOLV_CONF) {
  76
  77                 struct stat own;
  78
  79                 /* Is it symlinked to our own uplink file? */
  80                 if (stat(path, &own) >= 0 &&
  81                     st->st_dev == own.st_dev &&
  82                     st->st_ino == own.st_ino)
  83                         return true;
  84         }
  85
  86         return false;
  87 }
  88
  89 int manager_read_resolv_conf(Manager *m) {
  90         _cleanup_fclose_ FILE *f = NULL;
  91         struct stat st;
  92         char line[LINE_MAX];
  93         unsigned n = 0;
  94         int r;
  95
  96         assert(m);
  97
  98         /* Reads the system /etc/resolv.conf, if it exists and is not
  99          * symlinked to our own resolv.conf instance */
 100
 101         if (!m->read_resolv_conf)
 102                 return 0;
 103
 104         r = stat("/etc/resolv.conf", &st);
 105         if (r < 0) {
 106                 if (errno == ENOENT)
 107                         return 0;
 108
 109                 r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
 110                 goto clear;
 111         }
 112
 113         /* Have we already seen the file? */
 114         if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
 115                 return 0;
 116
 117         if (file_is_our_own(&st))
 118                 return 0;
 119
 120         f = fopen("/etc/resolv.conf", "re");
 121         if (!f) {
 122                 if (errno == ENOENT)
 123                         return 0;
 124
 125                 r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
 126                 goto clear;
 127         }
 128
 129         if (fstat(fileno(f), &st) < 0) {
 130                 r = log_error_errno(errno, "Failed to stat open file: %m");
 131                 goto clear;
 132         }
 133
 134         if (file_is_our_own(&st))
 135                 return 0;
 136
 137         dns_server_mark_all(m->dns_servers);
 138         dns_search_domain_mark_all(m->search_domains);
 139
 140         FOREACH_LINE(line, f, r = -errno; goto clear) {
 141                 const char *a;
 142                 char *l;
 143
 144                 n++;
 145
 146                 l = strstrip(line);
 147                 if (IN_SET(*l, '#', ';', 0))
 148                         continue;
 149
 150                 a = first_word(l, "nameserver");
 151                 if (a) {
 152                         r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
 153                         if (r < 0)
 154                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);
 155
 156                         continue;
 157                 }
 158
 159                 a = first_word(l, "domain");
 160                 if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
 161                         a = first_word(l, "search");
 162                 if (a) {
 163                         r = manager_parse_search_domains_and_warn(m, a);
 164                         if (r < 0)
 165                                 log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
 166                 }
 167
 168                 log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
 169         }
 170
 171         m->resolv_conf_mtime = timespec_load(&st.st_mtim);
 172
 173         /* Flush out all servers and search domains that are still
 174          * marked. Those are then ones that didn't appear in the new
 175          * /etc/resolv.conf */
 176         dns_server_unlink_marked(m->dns_servers);
 177         dns_search_domain_unlink_marked(m->search_domains);
 178
 179         /* Whenever /etc/resolv.conf changes, start using the first
 180          * DNS server of it. This is useful to deal with broken
 181          * network managing implementations (like NetworkManager),
 182          * that when connecting to a VPN place both the VPN DNS
 183          * servers and the local ones in /etc/resolv.conf. Without
 184          * resetting the DNS server to use back to the first entry we
 185          * will continue to use the local one thus being unable to
 186          * resolve VPN domains. */
 187         manager_set_dns_server(m, m->dns_servers);
 188
 189         /* Unconditionally flush the cache when /etc/resolv.conf is
 190          * modified, even if the data it contained was completely
 191          * identical to the previous version we used. We do this
 192          * because altering /etc/resolv.conf is typically done when
 193          * the network configuration changes, and that should be
 194          * enough to flush the global unicast DNS cache. */
 195         if (m->unicast_scope)
 196                 dns_cache_flush(&m->unicast_scope->cache);
 197
 198         /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
 199          * might now talk to a very different DNS server that just happens to have the same IP address as an old one
 200          * (think 192.168.1.1). */
 201         dns_server_reset_features_all(m->dns_servers);
 202
 203         return 0;
 204
 205 clear:
 206         dns_server_unlink_all(m->dns_servers);
 207         dns_search_domain_unlink_all(m->search_domains);
 208         return r;
 209 }
 210
 211 static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
 212         assert(s);
 213         assert(f);
 214         assert(count);
 215
 216         if (!dns_server_string(s)) {
 217                 log_warning("Out of memory, or invalid DNS address. Ignoring server.");
 218                 return;
 219         }
 220
 221         /* Check if the DNS server is limited to particular domains;
 222          * resolv.conf does not have a syntax to express that, so it must not
 223          * appear as a global name server to avoid routing unrelated domains to
 224          * it (which is a privacy violation, will most probably fail anyway,
 225          * and adds unnecessary load) */
 226         if (dns_server_limited_domains(s)) {
 227                 log_debug("DNS server %s has route-only domains, not using as global name server", dns_server_string(s));
 228                 return;
 229         }
 230
 231         if (*count == MAXNS)
 232                 fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
 233         (*count)++;
 234
 235         fprintf(f, "nameserver %s\n", dns_server_string(s));
 236 }
 237
 238 static void write_resolv_conf_search(
 239                 OrderedSet *domains,
 240                 FILE *f) {
 241         unsigned length = 0, count = 0;
 242         Iterator i;
 243         char *domain;
 244
 245         assert(domains);
 246         assert(f);
 247
 248         fputs("search", f);
 249
 250         ORDERED_SET_FOREACH(domain, domains, i) {
 251                 if (++count > MAXDNSRCH) {
 252                         fputs("\n# Too many search domains configured, remaining ones ignored.", f);
 253                         break;
 254                 }
 255                 length += strlen(domain) + 1;
 256                 if (length > 256) {
 257                         fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
 258                         break;
 259                 }
 260                 fputc(' ', f);
 261                 fputs(domain, f);
 262         }
 263
 264         fputs("\n", f);
 265 }
 266
 267 static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 268         Iterator i;
 269
 270         fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 271               "#\n"
 272               "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
 273               "# all known uplink DNS servers. This file lists all configured search domains.\n"
 274               "#\n"
 275               "# Third party programs must not access this file directly, but only through the\n"
 276               "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 277               "# replace this symlink by a static file or a different symlink.\n"
 278               "#\n"
 279               "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 280               "# operation for /etc/resolv.conf.\n"
 281               "\n", f);
 282
 283         if (ordered_set_isempty(dns))
 284                 fputs("# No DNS servers known.\n", f);
 285         else {
 286                 unsigned count = 0;
 287                 DnsServer *s;
 288
 289                 ORDERED_SET_FOREACH(s, dns, i)
 290                         write_resolv_conf_server(s, f, &count);
 291         }
 292
 293         if (!ordered_set_isempty(domains))
 294                 write_resolv_conf_search(domains, f);
 295
 296         return fflush_and_check(f);
 297 }
 298
 299 static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
 300         fputs_unlocked("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
 301                        "#\n"
 302                        "# This is a dynamic resolv.conf file for connecting local clients to the\n"
 303                        "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
 304                        "# configured search domains.\n"
 305                        "#\n"
 306                        "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
 307                        "# currently in use.\n"
 308                        "#\n"
 309                        "# Third party programs must not access this file directly, but only through the\n"
 310                        "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
 311                        "# replace this symlink by a static file or a different symlink.\n"
 312                        "#\n"
 313                        "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
 314                        "# operation for /etc/resolv.conf.\n"
 315                        "\n"
 316                        "nameserver 127.0.0.53\n", f);
 317
 318         if (!ordered_set_isempty(domains))
 319                 write_resolv_conf_search(domains, f);
 320
 321         return fflush_and_check(f);
 322 }
 323
 324 int manager_write_resolv_conf(Manager *m) {
 325
 326         _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
 327         _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
 328         _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
 329         int r;
 330
 331         assert(m);
 332
 333         /* Read the system /etc/resolv.conf first */
 334         (void) manager_read_resolv_conf(m);
 335
 336         /* Add the full list to a set, to filter out duplicates */
 337         r = manager_compile_dns_servers(m, &dns);
 338         if (r < 0)
 339                 return log_warning_errno(r, "Failed to compile list of DNS servers: %m");
 340
 341         r = manager_compile_search_domains(m, &domains, false);
 342         if (r < 0)
 343                 return log_warning_errno(r, "Failed to compile list of search domains: %m");
 344
 345         r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
 346         if (r < 0)
 347                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 348
 349         (void) __fsetlocking(f_uplink, FSETLOCKING_BYCALLER);
 350         (void) fchmod(fileno(f_uplink), 0644);
 351
 352         r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
 353         if (r < 0)
 354                 return log_warning_errno(r, "Failed to open private stub-resolv.conf file for writing: %m");
 355
 356         (void) __fsetlocking(f_stub, FSETLOCKING_BYCALLER);
 357         (void) fchmod(fileno(f_stub), 0644);
 358
 359         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
 360         if (r < 0) {
 361                 log_error_errno(r, "Failed to write private resolv.conf contents: %m");
 362                 goto fail;
 363         }
 364
 365         if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0) {
 366                 r = log_error_errno(errno, "Failed to move private resolv.conf file into place: %m");
 367                 goto fail;
 368         }
 369
 370         r = write_stub_resolv_conf_contents(f_stub, dns, domains);
 371         if (r < 0) {
 372                 log_error_errno(r, "Failed to write private stub-resolv.conf contents: %m");
 373                 goto fail;
 374         }
 375
 376         if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0) {
 377                 r = log_error_errno(errno, "Failed to move private stub-resolv.conf file into place: %m");
 378                 goto fail;
 379         }
 380
 381         return 0;
 382
 383 fail:
 384         (void) unlink(PRIVATE_UPLINK_RESOLV_CONF);
 385         (void) unlink(temp_path_uplink);
 386         (void) unlink(PRIVATE_STUB_RESOLV_CONF);
 387         (void) unlink(temp_path_stub);
 388
 389         return r;
 390 }